Methods and apparatus for the magnetic separation of fine parts



A nl 21, 1970 as. CASNER ETAL 3,507,389

METHODS AND APPARATUS FOR THE MAGNETIC SEPARATION OF FINE PARTS Filed Aug. 31, 1967 2 Sheets-Sheet l PREPARE SEMICODUCTOR SLICE I [ETCH GRID LINES ON SURFACE OF SLICEJ INSPECT SURFACE OF sucEI [MARK DEFECTIVE AREAS WITH MAGNETIC FLUID] I DRY MAGNETIC FLUID I I LSCRIBE AND BREAK SLICE INTO WAFERS] I SEPARATE WAFERS MAGNETICALLY I INVENTORS B. a. CA SNER R. r GOULSTONE April 21, 1970 B. G. CASNER ET AL 3,507,389

METHODS AND APPARATUS FQR THE MAGNETIC SEPARATION OF FINE PARTS 2 Sheets-Sheet 2 Filed Aug. 31, 1.967

United States Patent 3,507,389 METHODS AND APPARATUS FOR THE MAGNETIC SEPARATION OF FINE PARTS Bernard G. Casner, Emmaus, and Ray T. Goulstone, Allentown, Pa., assignors to Western Electric Company, Incorporated, New York, N.Y., a corporation of New York Filed Aug. 31, 1967, Ser. No. 664,704 Int. Cl. B03c 1/02 US. Cl. 209111.8 4 Claims ABSTRACT OF THE DISCLOSURE Improved apparatus are provided for the magnetic separation of a plurality of small parts. The parts, some of which have been magnetically identified, are conveyed under a magnet, whose pole pieces are shaped at their ends to form inwardly pointing, sharp edges, whereby the lines of flux are intensified and confined to a comparatively narrow region across the orifice of a recessed space between the pole pieces. The magnetically identified parts, but no others, are accelerated through the orifice into the recessed space.

BACKGROUND OF THE INVENTION Field of the invention This invention generally relates to the magnetic separation of a plurality of small parts. The invention is especially concerned with magnetically sorting a plurality of small parts that have been formed from a single sheet of nonmagnetic material, which sheet, prior to its subdivision, has been inspected and defective regions have been given a magnetic identificationv Accordingly, this invention provides improved apparatus that will facilitate the magnetic separation of small, magnetically identified parts formed from defective regions of a sheet of material from the remaining small parts not so magnetically identified.

While this invention may fined utility in many magnetic separation operations, it is described herein for convenience and clarity with specific regard to the manufacture of semiconductor waters, which is particularly illustrative of the practice of the invention. It is to be understood, however, that the invention is not intended to be limited to the manufacture of semiconductor wafers.

Description of the prior art It is conventional in the manufacture of semiconductive devices to subdivide a crystal of semiconductive material into a series of thin discs by making a number of parallel slices through the crystal. These discs, referred to herein as slices, are given appropriate treatment, as by doping with desired conductivity-type determining impurities, and each slice is then divided into many hundreds of small chips or wafers. The slices are usually comprised of a rather hard, brittle material, such as silicon or germanium, thus the shaping of the wafers is readily accomplished by one of several known methods, such as by sawing the slice with a diamond-impregnated wheel, or by mounting the slice on a ruling engine, scribing fine lines with a diamond stylus, and then breaking the slice along the scribed lines. In addition, ultrasonic cutting or grit-blasting techniques are used in some applications.

Prior to the final assembly of the wafers, it is necessary to inspect the wafers visually and electrically in order to identify and discard wafers that fail to meet physical and electrical specifications. Because semiconductor wafers are quite small (a typical wafer may be "ice only 4 to 6 mils in thickness and 20 mils square), manipulation of the individual wafers during inspection and testing operations is a tedious process. For this reason, it has been found economically desirable to inspect selected areas of a slice from which semiconductor wafers will be formed, prior to the time that the slice is divided into individual wafers.

Typically, a pattern of grid lines is etched through the oxide layer on the surface of a slice to outline the specific areas of the slice from which each individual wafer will be formed. Sensing elements of a test probe are then sequentially indexed from one area to the next, and certain of the electrical properties are measured. At the same time, or during a separate operation, an operator may inspect each outlined area under a microscope for any visible flaws that may appear on the surface of the slice.

When defective areas on the surface of the slice are detected, these areas are identified with a mark or symbol so that, after the slice has been divided into individual wafers, the wafers formed from these defective areas can be separated from the remaining wafers that have passed inspection.

One method of identifying the defective wafers utilizes a marking instrument, such as a pen, that, upon signal from the operator or the electrical testing device, will cause a small drop of ink to be deposited on the defective area. The ink mark provides a visual identification that enables an operator to sort out the defective wafers.

It will be appreciated that considerable dexterity is required in manual sorting of the wafers since it is possible to produce several thousand wafers from a single slice only an inch or two in diameter. In examining the wafers, the operator must view the wafers under magnification and use a vacuum pencil or similar device to pick up and manipulate each wafer. Further, since an ink dot is deposited on only one side of a defective wafer, the wafers must be turned over and examined on both sides to insure a thorough inspection.

It is understandable that the accuracy of this tedious and time-consuming sorting process is subject to human error and that some defective wafers may pass undetected. Therefore, in order to increase both the speed and efliciency of the sorting operation, it has been suggested that defective areas of a slice be identified by depositing a small dot of magnetic particles on the surface of the slice. Then, after the slice has been divided into wafers, those wafers formed from defective areas can be separated from the others by magnetic means.

In a related, commonly assigned, copending application Ser. No. 664,703, filed Aug. 31, 1967, we, together with coapplicant P. R. Hance, have described a system especially adapted for marking defective areas of a slice with a magnetic fluid. Briefly, this system includes an improved magnetic marking fluid of controlled viscosity that is comprised of a suspension of magnetic particles in a substantially nonvolatile carrier liquid, especially adapted to be fed by means of an improved marking device. Upon signal, a metered amount of the magnetic fluid is expressed from the marking device and is deposited with precision over selected areas on the surface of the slice.

In a preferred embodiment of that invention, the operation of the marking device and the delivery of the magnetic fluid are controlled by signals received from electrical testing apparatus so that a dot of magnetic fluid will automatically be deposited at any point on the surface of the material that is found to be defective by the electrical testing apparatus.

In distinction to this related patent application, the instant invention relates to methods and apparatus for 3 magnetically separating the wafers after the slice has been inspected, its defective areas have been magnetically identified, and the slice has been divided into a plurality of wafers.

SUMMARY OF THE INVENTION Accordingly, it is an object of this invention to provide improved apparatus for separating a plurality of small parts in accordance with their magnetic properties.

Another object of this invention is to provide an improved magnetic separation device that is especially designed to facilitate the magnetic separation of a plurality of small parts.

A more specific object of this invention is to provide an improved device that will magnetically separate any defective parts that have been identified with a small dot of magnetic material from the remaining parts that are free from defects and have not been so magnetically identified.

Briefly, these and other objects of this invention are achieved by conveying the small parts to be sorted under the pole pieces of a magnet, which pole pieces are shaped at their ends to form inwardly pointing, sharp edges whereby the lines of flux are intensified and confined to a comparatively narrow region across the orifice of a recessed space. This region is approximately defined by the inwardly pointing ends of the pole pieces and by the edges of the recessed space, located between the pole pieces that is adapted to receive small parts. By confining the lines of flux to this narrow region, the small parts magnetically identified are accelerated past the ends of the pole pieces and most of them will be deposited within the recessed space and will not interfere with further sorting operations.

BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a schematic flow sheet illustrating the general background of this invention;

FIGURE 2 is a perspective view of the magnetic separating device of this invention; and

IFGURE 3 is a sectional view, partially cut away, taken along line 3-3 of FIGURE 2.

DETAILED DESCRIPTION Referring first to FIGURE 1, a semiconductor slice is initially prepared by cutting it from a large crystal usually grown from a Group IV element sush as silicon or germanium. After the slice is carefully cleaned and polished, it is doped with materials such as elements from Groups III and IV of the Periodic Table to establish the desired number and types of p-n junctions. The slice is now suitably prepared for subdivision into a number of wafers from which semiconductors, such as diodes and transistors, can be manufactured.

While the slice is being doped, a hard oxide layer may be formed on its surface. Because the oxide layer is resistant to normal scribing apparatus, such as a diamond stylus, it has been found convenient to etch a grid line pattern, as with hydrofluoric acid, on the surface of the slice. Thus, not only is the hard oxide layer removed for cutting purposes, but also the individual areas of the surface of the slice from which each wafer is to be formed are clearly identified for inspection.

After the grid lines have been etched, the slice is mounted on testing apparatus so that it may be contacted with test probes. The slice is indexed under the probes and, at each indexed position, a separate grid area (representing a future wafer) is contacted with the test probes for measurement of certain electrical properties. At this or a later time, each distinct area is also inspected for visible flaws under a microscope. Any area failing to pass inspection is identified by marking it with a fluid suspension of magnetic particles (hereinafter referred to for convenience as a magnetic fluid).

After the inspection of the surface of the slice is complete, the slice is set aside, preferably in an oven, to

allow the magnetic fluid to dry to a relatively hard, nonsmearing solid. When the magnetic fluid is dry, the slice is mounted on a ruling engine and fine lines are scribed within the etched grid lines by using a fine diamond-tipped stylus. The slice is then broken into individual wafers by any convenient technique such as by placing the slice between two thin sheets of flexible material and then running a small roller back and forth over the surface of the enclosed slice.

After the individual wafers have been formed, they are sieved to remove random small chips and pieces as well as any oversize material. The middlings are then subjected to the influence of a magnetic field and the inferior wafers containing a deposit of magnetic particles are separated from thOSe wafers that have passed inspection.

The details of the construction of the magnetic separation device 10 are shown in FIGURE 2. The device 10 is comprised of a base plate 11 upon which is mounted a vibrator 12. The vibrator 12 may be of any common type, such as an electrically operated one. The vibrator 12 is joined to a vibratory track 13 by means of an adjustable linkage 14.

The vibratory track 13 is supported for substantially horizontal reciprocation by resilient supports 15-15 so that the combination of the plate 11, the vibrator 12, the track 13 and the linkage 14 forms a vibratory conveyor. As illustrated in the drawing, the resilient supports 15-15 are compirsed of upstanding legs made from, for example, spring steel. Other commonly known expedients for mounting the vibratory track 13 can be used, such as by substituting resilient mounting pads made of an elastomeric material for the supports 15-15. The side portions of the vibratory track 13 are defined by restraining side wall members 16-16.

A magnetic pickup device 17 is removably mounted on a support 18 in such a manner that the device 17 bridges, either at an angle or straight across, the entire width of the vibratory track 13. A receiving tray 19 is removably positioned upon the base plate 11 adjacent the discharge end of the vibratory track 13. As illustrated in the drawing, a plurality of small wafers 20-20' are distributed over the vibratory track 13. The vibratory track 13, when energized by the vibrator 12, is adapted to convey the wafers 20-20 from the end of the track 13 adjacent the vibrator 12 toward the receiving tray 19.

The details of the construction of the magnetic pickup device 17 are shown in FIGURE 3. Here it can be seen that the magnetic pickup device 17 is comprised of a pair of magnets 21-21, which could readily be one magnet, that are associated with corresponding pole pieces 22-22. The lower ends of the pole pieces 22 are bent sharply inward toward each other to form foot portions 23-23. The foot portions 23-23 are provided with flat, horizontol undersurfaces 24-24 spaced slightly above the vibratory track 13. The feet 23-23 terminate in inwardly facing sharp edges 25-25 that are spaced in close proximity to each other along the edges of an orifice in a receptacle or a recessed space 27.

The small space between the foot portions 23-23 of the pole pieces 22-22 and the magnets 21-21 are filled with a filling material 26. The nature of this compound 26 is not of particular importance so long at it may be easily formed and will not adversely affect the magnetic properties of the magnetic pickup device 17. Conveniently, this filling material 26 may be comprised of a potting compound such as an epoxy resin. In the region between the edges of the feet 25-25, the recessed spaced 27 is formed in the filling compound 26 for the full length of the pickup device 17. This recessed space 27, as will be discussed more fully below, serves as a receiver for any defective wafers that are attracted by the magnetic pickup device 17.

In operation, a number of wafers 20-20 are deposited upon the vibratory track 13 adjacent the end attached to the vibrator. Previously, these wafers have been inspected while in the form of a slice, the defective wafers have been identified with a dot of magnetic particles, and the slice has been subdivided into individual wafers 2020.

The vibrator 12 is now energized and, acting through linkage 14, the vibratory track 13 is caused to reciprocate or vibrate and convey the wafers 2020 along the track 13 (from left to right, as illustrated in the drawing) until they pass under the magnetic pickup 17.

The action of the magnetic pickup 17 is schematically illustrated in FIGURE 3 where wafers 2020 can be seen moving under the influence of the pole pieces 2222. Due to the fact that the feet 2323 of the pole pieces 2222 are pointed inwardly and terminate along sharp edges 25- 25, the effects of the magnetic lines of flux are substantially confined to the horizontal region that lies between and is immediately adjacent the edges 2525 and crosses the orifice of the recessed space 27. By these means, when the wafers 2020 are conveyed under the upstream pole piece 22, the wafers shown at 20a20a bearing magnetic identification are influenced by the magnetic lines of flux but not sufficiently as to be lifted olf the vibratory track 13. However, when the wafers move into the region below the edges 2525, the magnetically identified wafers shown at 20b20b are strongly attracted by the magnetic lines of flux and rapidly accelerate through the region between the edges 2525. The rapid acceleration of these magnetically identified wafers shown at 20b20b causes most of them to be carried by their own inertia into the recessed space 27 and come to rest therein. Since the defective wafers are deposited in this recessed space 27, they are out of the way and will not interfere with the processing of further wafers.

If the material 26 filled the space between the specially shaped feet 23-23 of the pole pieces 2222, the defective wafers would attach themselves, somewhat randomly, to the undersurfaces 24-24 of the pole pieces 2222 near the edges 2525 and build up accretions of wafers. If these accretions were permitted to accumulate on the pole pieces 2222, they would eventually fall, or otherwise be dislodged from the pole pieces, and become remixed with the good wafers shown at 20c20c. Further, since the accretions would hang down from and bridge the underside of the pole pieces 2222, their accumulation would reduce the clearance between the vibratory track 13 and the underside of the pole pieces 2222 and block or otherwise interfere with the free passage of subsequent wafers under the magnet. Understandably, either result would reduce the accuracy and etficiency of the sorting operation.

The good wafers shown at 20c-20c that bear no magnetic identification and pass freely under the magnetic pickup device 17 without interference and continue on their way until they fall into a receiving tray 19 in which they are accumulated for further processing.

After a substantial number of defective wafers have accumulated shown at 20b in the recessed space 27, they may be easily removed by lifting the magnetic pickup device 17 off of its support 18 and dislodging the magnetically adhered wafers by brushing out the space 27.

From the foregoing, it will be understood that the device of this invention makes it possible accurately to sort a plurality of parts according to their magnetic identification with considerable ease and facility.

Although certain embodiments of the invention have been shown in the drawings and described in the specification, it is to be understood that the invention is not limited thereto, is capable of modification and can be rearranged without departing from the spirit and scope of the invention.

What is claimed is:

1. In an apparatus for the magnetic separation of a plurality of small parts in which the small parts are conveyed under the pole pieces of a magnet and the parts having magnetic characteristics are attracted thereto, the improvement comprising pole pieces having food portions turned inwardly toward each other and terminating in relatively sharp edges, whereby the magnetic lines of flux are substantially confined within a region immediately adjacent the edges of the pole pieces and having a recessed space between the edges of the pole pieces that is adapted to receive parts having magnetic characteristics.

2. Apparatus according to claim 1, wherein the recessed space is formed in a non-magnetic material contiguous with the pole pieces of the magnet.

3. Apparatus according to claim 1, wherein the recessed space is elongated and each pole piece of the magnet extends along each of the long edges of the recessed space so that the magnetic flux crosses the opening of the recessed area.

4. In an apparatus for the magnetic separation of a plurality of small parts, some of which have been identified by the application of a magnetic substance, in accordance with their identification, in which the small parts are conveyed by on a track beneath the pole pieces of a magnet and the parts having the magnetic identification are attracted thereto, the improvement comprising:

pole pieces having foot portions turned inwardly toward each other and terminating in relatively sharp edges, whereby the magnetic lines of flux are substantially confined within a region immediately adjacent the edges of the pole pieces and such flux is maximized in a plane formed by the edges of said pole pieces, such plane being substantially parallel to said track; and

a non-magnetic material contiguous with the foot portions of said pole pieces and having receptacle formed therein for collecting the magnetically identified parts, the orifice of such receptacle lying in the plane of maximum flux.

References Cited UNITED STATES PATENTS 1,360,601 11/1920 Ullrich 209-214 1,565,038 12/1925 Ullrich 209-223 2,724,504 11/1955 Blind 209223 RICHARD A. SCHACHER, Primary Examiner US. Cl. X.R. 209115, 223 

